The present invention relates to a navigation system which displays the present position of a vehicle on a course of travel course from a start position to a destination.
This kind of navigation system is exemplified by Japanese Patent Application Unexamined Open No. Sho 56-74798.
In the above-identified document, a conventional navigation system employs a display unit. After an operator (driver) selects both a start point and a destination point from a diagrammatic roadmap image on the screen, the system searches map data stored on a memory for the most suitable course and then displays that course on the roadmap displayed on the screen. When the vehicle on which the navigation system is mounted starts to move, the system derives the present position of the vehicle from the orientation of the vehicle detected by means of a magnetic direction sensor and the displacement of the vehicle by means of a distance sensor and displays the continuously changing present position of the vehicle on the roadmap.
However, the conventional navigation system has the drawbacks described below. Since the calculation of the present position of the vehicle is carried out on the basis of the output signals from a magnetic direction sensor which relies on geomagnetism, the present position of the vehicle will incur errors due to inaccurate detection of direction when the vehicle moves through magnetic disturbances.
This will be especially significant on freeways. The ferromagnetic reinforced concrete structure of the freeway itself adversely effects geomagnetic devices of all sorts. The same problem arises when the vehicle crosses bridges or passes through tunnels. In such cases, the measured travel distance will also tend to exceed the real travel distance, and all these errors add up as the vehicle approaches its destination. Consequently, the guide display on the displayed roadmap can be inaccurate enough to actually be misleading.